Inductive power distribution system

ABSTRACT

Inductive power transfer across an extended gap ( 100 ) from a primary conductor ( 119 ) is provided by means of a resonant intermediate loop comprised of capacitor ( 118 ) with inductor ( 117 ) carrying a larger resonating current, that can in turn generate an inductive field to be collected by a pickup coil ( 120 ). This process and device find application in an electroluminescent advertising panel.

RELATED APPLICATIONS

[0001] This application is a division of co-pending application Ser. No.09/423,364, filed on Nov. 8, 1999, which is the national phase of PCTInternational Application No. PCT/NZ98/00053, filed on May 4, 1998,under 35 U.S.C. § 371. The entire contents of each of theabove-identified applications are hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

[0002] This invention relates to a power supply for anelectroluminescent display, and to the use of inductive power transferto provide power across an extended gap between a primary inductivetrackway and a secondary pickup device, for a range of purposesincluding motive power, battery charging and lighting, includinglighting using electroluminescent panels.

BACKGROUND

[0003] Inductive power transfer, although contactless, has in mostapplications in the prior art using primary pathways required that theconfigurations shall include ferromagnetic cores and that the secondaryor pickup shall be quite closely placed in proximity to, or about theprimary conductor. For example, Kelley in U.S. Pat. No. 4,833,337 useselongated ferrite inverted “U” cores and a ferrite member fixed to theprimary pathway as well. BOYS & Green (WO92/17929) use “E” cores withone primary conductor located inside each space between the three limbsof the “E”. Bolger (U.S. Pat. No. 3,914,562) teaches a 120 Hz primaryinductive cable along a roadway, the cable having iron laminations alongits entire length. These laminations face corresponding laminationswithin the moving vehicles that draw power from the tracks. These areexpensive, heavy constructions which will exhibit magnetic attractionforces and any magnetostrictive effects within the cores will tend tocause noise. For transferring power to moving road vehicles, avoidanceof core structures (at least in the primary pathway) and a widertolerance in positioning is clearly useful.

[0004] Inductive power transfer systems in which various portions of thesystem are tuned to resonance are somewhat liable to instability shouldone or more resonant circuits assume a different resonant frequency tothat of the system mean. Means to enhance stability are always useful,given that resonance is in most cases the preferred way to optimise thetransfer of inductive power.

[0005] There are many applications in attention-gathering fields (i.e.advertising) in which it will be useful to extend the gap over which auseful field can be transmitted under inductive power transferprinciples. Advantages of doing this include the concealment of thepower sources so that panels appear to magically light up without avisible connection. Hence the use of inductive power transfer, whichitself may involve higher frequencies, as a way of drivingelectroluminescent panels across a gap and without bare wires orcontacts is a useful venture.

[0006] Electroluminescent panels have been available since at least 1957as a source of lighting or of display and advertising material, yet theyhave proven to be difficult to drive at an acceptable level ofbrightness and at the same time retain a reasonably long life. Panelsrequire a relatively high frequency (well above mains frequency) inorder to glow at a useful level. Prior-art driving circuits such asdedicated chips rely on inverters to develop AC power at typically800-200 Hz, and up to typically 50 V peak-to-peak. Because the output ofthose inverters is substantially a square-wave waveform the phosphors ofthe panels are not excited optimally and brightness is not remarkable.Attempts to get more light with higher driving voltage usually resultsin breakdown of the dielectric and a failure of the panel, or a markedlycurtailed life. There may be thermal runaway effects involved.

OBJECT

[0007] It is an object of this invention to provide an improved way todrive loads such as (but not limited to) electroluminescent panelsacross a gap using inductively transferred electric power, or at leastto provide the public with a useful choice.

STATEMENT OF THE INVENTION

[0008] In a first broad aspect the invention provides means forinductive power transfer across an extended gap between a primaryconductor and a secondary resonant pickup circuit, the means comprisingan intermediate resonant loop, resonant at a system-wide resonantfrequency and capable of being positioned within an inductive powertransfer system so that inductive power is capable of being coupledinductively from the primary inductive conductor through theintermediate resonant loop to the at least one secondary resonant pickupcircuit capable of collecting the inductive power.

[0009] Preferably the invention provides means for coupling inductivepower as described in this section, wherein the intermediate resonantloop comprises a capacitance and an inductance, together resonant at thesystem-wide resonant frequency.

[0010] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the inductance maycomprise at least one lumped inductance ad at least one extendedinductance.

[0011] Optionally the at least one element having inductance within theintermediate resonant loop may comprise at least one intermediate lumpedinductance comprised of a sub-loop having one or more turns and at leastone extended intermediate inductance being the inductance of the loopitself.

[0012] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the at least onelumped inductance is capable of receiving inductive power from a primaryconductor.

[0013] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the intermediateresonant loop is extended over a lateral distance so that one or more,spaced-apart, secondary resonant circuits may draw power from theintermediate resonant loop.

[0014] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the intermediateresonant loop includes means to limit the amount of resonating currentflowing.

[0015] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the means to limitthe amount of resonating current flowing includes means for at leastpartial decoupling of the intermediate loop from the primary conductor.

[0016] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the secondaryresonant circuit provides motive power to an electrically poweredvehicle.

[0017] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the intermediateresonant circuit is extended over a lateral distance beneath at least apart of a route taken by a vehicle, so that the intermediate resonantcircuit is capable of providing power to the vehicle when the vehicle issituated adjacent to the position of the intermediate resonant circuit.

[0018] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the intermediateresonant circuit provides a charging current to one or more batteryunits within a vehicle when the vehicle is situated adjacent to theposition of the intermediate resonant circuit, such as at a bus stop.

[0019] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the intermediateresonant circuit provides frequency stability to an inductively poweredsystem.

[0020] Preferably the invention provides means for coupling inductivepower as claimed in the preceding claim, wherein the intermediateresonant circuit includes active frequency-adjusting means or the liketo overcome any system instability that may arise.

[0021] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the intermediateresonant circuit provides a charging current to one or more batteryunits.

[0022] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the secondaryresonant circuit provides electric power to a light source.

[0023] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the electricallypowered light source is an electroluminescent panel driven withsubstantially sine-wave alternating current at an effective voltage andat an effective frequency.

[0024] Preferably the invention provides for coupling inductive power asdescribed elsewhere in this section, wherein the intermediate resonantloop comprises at least one element having inductance and at least oneelement having capacitance, together forming a circuit resonantsubstantially at the system-wide resonant frequency, and in which theintermediate resonant loop is capable of intercepting an inductive fieldfrom the primary inductive pathway and thereby having an effectiveresonating current induced within it, the inductive field developedabout the intermediate resonant loop being in turn capable of inducing acurrent within the secondary resonant circuit.

[0025] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the at least oneelement having inductance within the intermediate resonant loopcomprises at least one lumped inductance comprised of a coil made from afraction of a turn to a plurality of turns, the or each lumpedinductance being situated at one position or, if the intermediateresonant loop has more than one lumped inductance, is situated atspaced-apart positions around the intermediate resonant loop, and atleast one extended intermediate inductance is comprised of theinductance of the loop itself.

[0026] Preferably the invention provides means for coupling inductivepower as described elsewhere in this section, wherein the orientation ofthe magnetic flux generated, when in use, about the intermediate lumpedinductance is capable of enhancing the transfer of inductive power atthat position. In another view, the inductive field developed, when inuse, about the intermediate resonant loop may be caused to beconcentrated at one or more predetermined positions by forming theintermediate resonant loop into a sub-loop having one or more turns atthe or each position, so that the transfer of inductive power into orout of the inductive field developed, when in use, about theintermediate resonant loop is enhanced at the one or more predeterminedpositions.

[0027] Preferably the invention provides means for coupling inductivepower over an extended distance from a primary, energised, inductivepathway having at least one conductor capable of carrying an alternatingcurrent having a system-wide consistent frequency, to a secondaryresonant circuit capable of collecting the inductive power, the meanscomprising an intermediate resonant loop including at least one elementhaving inductance and at least one element having a capacitance,together resonant at the system-wide resonant frequency, theintermediate resonant loop being capable of intercepting an inductivefield from the primary inductive pathway and thereby having a currentinduced within it, the inductive field developed, when in use, about theintermediate resonant loop being in turn capable of inducing a currentwithin the secondary resonant circuit so that in use it develops asubstantial resonating current.

[0028] Preferably the lumped inductance is capable of receivinginductive power from a primary conductor, optionally connected to aprimary lumped inductance adjacent to the intermediate lumpedinductance.

[0029] Optionally the intermediate resonant loop may be extended over alateral distance so that one or more, spaced-apart, secondary resonantcircuits may draw power from the intermediate resonant loop.

[0030] Alternatively, the invention provides means to extend thedistance between a source of changing magnetic fields and a magneticfield pickup means (including a secondary resonant circuit) over whichan effective transfer of inductive power can take place, the meanscomprising a intermediate resonant circuit placed about the source ofchanging magnetic fields so that in use it develops a substantialresonating current.

[0031] Preferably the intermediate resonant loop of the invention isprovided as an accessory to be overlaid upon a primary resonant pathway,so that it is capable of intercepting at least some of the magnetic fluxsurrounding one or more conductors of the primary pathway.

[0032] Alternatively the intermediate resonant loop of the invention maybe provided as an accessory to be laid over or about a secondaryresonant circuit or pathway so that the loop collects a magnetic fluxand forwards it to the secondary resonant circuit or pathway.

[0033] Optionally the invention provides means to improve the frequencystability of an inductive power transfer system including a primary,energised, inductive pathway having at least one conductor capable ofcarrying an alternating current having a system-wide consistentfrequency, and one or more secondary resonant circuits capable ofcollecting the inductive power, the means comprising an intermediateresonant loop including at least one element having inductance and atleast one element having a capacitance, together resonant at thesystem-wide resonant frequency, the intermediate resonant loop beingcapable of intercepting an inductive field from the primary inductivepathway and thereby having a current induced within it, the inductivefield developed, when in use, about the intermediate resonant loop beingin turn capable of inducing a current within the secondary resonantcircuit so that in use it develops a substantial resonating current,wherein the stabilizing property depends on the reversal of the effectof a detuning or destabilising event as it crosses an inductive linkbetween conductors, so that the addition of a second inductive linkinherent in the use of an intermediate link between a primary and asecondary circuit causes the destabilising effect to be reversed twice.

[0034] In a second broad aspect the invention provides a power supplycapable of removing its electric power through an intermediate resonantloop as described elsewhere in this section, the power supply beingcapable of generating a substantially sine-wave alternating current atan effective voltage and an effective frequency for use with acapacitative load such as one or more electroluminescent panels; thepower supply being itself supplied with electric power through anintermediate loop as described elsewhere in this section.

[0035] Preferably the invention provides a power supply as describedelsewhere in this section, the power supply including a pair ofswitching devices driven in a complementary manner at the effectivefrequency, and includes an inductor capable of resonance at or about theeffective frequency when connected to the capacitative load.

[0036] Preferably the invention provides a power supply as describedelsewhere in this section, wherein the one or more electroluminescentpanels are connected in parallel with a frequency-adjusting capacitance.

[0037] Preferably the invention provides a power supply as describedelsewhere in this section, wherein the power supply includes a circuitcomprising a first inductor of large inductance in series between apower supply and a centre tap of a second, resonating inductor of largeinductance, the start and finish of the winding of the second inductorbeing connected to a second terminal of each of a pair of switchesincluding current amplification properties, each first terminal of eachswitch being connected to the return line to the power supply, and eachcontrol terminal of each switch being connected via a resistor chain tothe second terminal of the other switch, and the capacitative load beingplaced between the second terminals of the two switches.

[0038] Optionally the invention provides a power supply as describedelsewhere in this section, wherein the circuit may be functionallyenabled or disabled by connecting an intermediate point in one or bothresistor chains to a potential capable of reversibly interruptingoscillation, thereby halting the supply of sine wave power.

[0039] Optionally the invention provides a beverage dispenser includingone or more electroluminescent panels within valves capable ofcontrolling the flow of the beverage, wherein the electroluminescentpanels are provided with power by a power supply as described elsewherein this section from an inductive power distribution system over aspace, the width of the space being enhanced by the inclusion of anintermediate resonant unit, so that the effective distance between theelectroluminescent panel and the source of inductive power may beincreased.

[0040] Preferably the invention provides a power supply wherein thepower supply includes a circuit comprising a first inductor of largeinductance in series between a power supply and a centre tap of asecond, resonating inductor of large inductance, the start and finish ofthe winding of the second inductor being connected to a second terminalof each of a pair of switches including current amplificationproperties, each first terminal of each switch being connected to thereturn line to the power supply, and each control terminal of eachswitch being connected via a resistor chain to the second terminal ofthe other switch, and the capacitative load being placed between thesecond terminals of the two switches.

[0041] Preferably the invention provides a power supply as describedelsewhere in this section, wherein the circuit may be functionallyenabled or disabled by connecting an intermediate point in one or bothresistor chains to a potential capable of reversibly interruptingoscillation, thereby halting the supply of sine wave power.

[0042] Optionally the power supply itself is supplied with electricpower without the intervention of an intermediate loop as describedelsewhere in this section.

[0043] Optionally the power supply itself is supplied with electricpower from a utility rather than from an inductive power transfersystem, in which case the power supply includes a circuit comprising afirst inductor of large inductance in series between a power supply anda centre tap of a second, resonating inductor of large inductance, thestart and finish of the winding of the second inductor being connectedto a second terminal of each of a pair of switches including currentamplification on properties, each first terminal of each switch beingconnected to the return line to the power supply, and each controlterminal of each switch being connected via a resistor chain to thesecond terminal of the other switch, and the capacitative load beingplaced between the second terminals of the two switches, but does notinclude a secondary resonant pickup circuit.

[0044] Optionally the invention provides a power supply as describedelsewhere in this section, wherein the power supply itself is suppliedwith electric power from a conventional utility supply and the resonantsecondary pickup aspect itself is absent.

[0045] Preferably the electric power is rectified.

DRAWINGS

[0046] The following is a description of a preferred form of theinvention, given by way of example only, with reference to theaccompanying diagrams.

[0047]FIG. 1: is a circuit diagram used to drive an electroluminescentpanel with inductively transferred power, using an intermediate loop.

[0048]FIG. 2: is a circuit diagram showing the intermediate loop orcircuit.

[0049]FIG. 3: is a prior-art diagram of a self-illuminated roadway stud,in section, when attached to a roadway.

[0050]FIG. 4: is a diagram of a self-illuminated roadway stud using anintermediate loop to continue the power supply connection after morelayers of seal have been applied.

[0051]FIG. 5: is a sectional diagram of a vehicle, driven withinductively transferred power and an intermediate loop as part of thetrackway.

[0052]FIG. 6: is a sectional diagram of a vehicle, driven withinductively transferred power and an intermediate loop included withinthe vehicle.

[0053]FIG. 7: is a circuit diagram used to drive an electroluminescentpanel with inductively transferred power, without using an intermediateloop.

PREFERRED EMBODIMENT APPLICATION EXAMPLE 1 Electroluminescent Panel

[0054] The invention will be described in relation to a particularapplication; driving an electroluminescent panel incorporated into thehandle of a beer tap as used in bars, where the panel serves as abackground for advertising material. Clearly, the invention can beapplied to other situations.

[0055] The circuit for driving an electroluminescent panel includes foursections: (a) DC to AC conversion producing a sine wave output at anoptimal frequency, capable of driving the panel more effectively, (b)power pickup means, (c) secondary control means operating on rectifiedpower, and (d) panel disabling means. Refer to FIG. 1.

[0056] Section (a) of the circuit is a DC to sine wave converter whichis adapted for a capacitative load such as that of an electroluminescentpanel 115. Electroluminescent panel 115 is a device having usually onetransparent conductive plane, a dielectric layer including one or morephosphors capable of emitting light when excited, and a secondconductive plane, so comprising a capacitor. The light emitted from anenergized panel could be regarded as a result of the existence of a“lossy dielectric” because a change in the voltage field between theplanes is required to excite the phosphor. The panels used are made withflexible plastics materials by the New Zealand manufacturer of theadvertising displays, using DuPont phosphors and chemicals. A typicalpanel has an area of about 50 cm² and a capacitance of about 10 nF.

[0057] We found that conventional excitation with approximately squarewave waveforms from purpose-designed integrated-circuit (IC) excitationdevices (e.g. the Supertex HV803) resulted in about 120 Lux of emittedlight, which is visible but not dramatic. Attempts to drive the panelsharder, with more voltage, in order to get more light resulted infailure of either or both the panel and the IC driver and perhaps thisis a result of the “impulse” nature of the step changes of a square wavedrive being poorly matched to the requirements of the phosphor in termsof perceptible light. Perhaps thermal runaway effects occur during thefailure process. In order to exercise the dielectric of a panel, and soexcite the phosphors as efficiently as possible, it seems preferable toapply an AC waveform having a constant rate of change, such as a sinewave. We have confirmed that panels driven with a sine wave alternatingcurrent using a circuit according to this invention can radiate aconsiderably increased amount of light over an apparently long periodbefore degradation or failure occurs. Accordingly, we have constructed asine-wave power supply circuit capable of producing a reasonably puresine wave when used with a capacitative load (includingelectroluminescent panels) yet having high conversion efficiency and alow parts count.

[0058] Referring to FIG. 1, 101 is a positive power input and 102 is anegative power input. 103 is an inductance of small physical size (total13×13×9 mm) wound from 3500 turns of 0.05 mm insulated copper wire on anE-I ferrite coil former. It has an inductance of about 1.2 Henry. Thispart converts a voltage source such as our secondary pickup coilcombination including a smoothing capacitor into a current source.

[0059] The active components in the circuit comprise 104 and 105, twoNPN transistors in a cross-coupled type of circuit. Suitable transistorsare rated for 200 V V_(ceo) 250 mA, and have a β of about 40. (Steeringdiodes 108 and 116 simply provide for circuit disabling—see section(d)). The emitters of the transistors are connected to supply line 102;their bases are connected at the first junction (between 106 and 107) ofa resistor chain comprising 106 (47K), 107 (82K), and 109 (330K); or110, 111, and 112 for the other transistor. The top ends of theseresistor chains are connected to the collectors of the oppositetransistors. Also connected between the collectors are the ends of 113,a center-tapped inductor otherwise like 103 which has the center tapconnected to 103, and optionally one or more capacitors such as 114,used for frequency-adjusting purposes to reach the about 1200 Hz desiredfrequency, and 115, one or more electroluminescent panels. Inductor 113serves as a resonating inductance and as a DC feed or splitter.

[0060] Note that the desired frequency is not related to the “firstfrequency” used for power distribution which is typically in the rangeof from 10 kHz up to perhaps 30 kHz, depending on switching deviceratings, harmonic considerations, and the like.

[0061] The operating frequency may be trimmed by adjustment of capacitor114 to run at about 1200 Hz with the about 10 nF preferredelectroluminescent panel. The magnetizing inductance of the centertapped inductor 113, which serves as a DC feed and as part of a resonantcircuit, is about 1.2 Hy. In use the circuit is fed at about 50 V DCwhich results in about 400 Lux of greenish light being emitted by thepanel 115. For special effects such as flashing, the circuit can beswitched off by disabling the base drives, actually by grounding thejunction of the diodes 116 and 108 so interrupting signal and basecurrent. On raising the diode voltage (above about 1.5 V, so that thediodes become non-conducting) the circuit rapidly (in about 2-3 cycles)builds up to its full amplitude of oscillation. The controller 128provides this output. (We have not discussed in detail any actualcontrol signals for enabling or disabling the circuit. These may beinternally generated by a simple free-running multivibrator, forexample, or may be picked up from an external source perhaps through thedemodulation of control signals carried either through the primaryconductor cable or as electromagnetic fields). Thus the circuit can beprogrammed to make the panel flash on and off, or be dimmed with a rapidenable/disable sequence. Brightness control is also possible by varyingthe applied voltage, as long as the ratings of the panel and other partsare not exceeded. The applied voltage could be varied by varying thereference voltage fed to the controller 127 for the shorting switch 124.Another mode of use of the comparator 128 is to inhibit thepanel-driving converter when the bus voltage is under 40 volts, orenable it when the voltage is over 40 V. This has the effect in thetarget application of causing the panel to flash brightly and perhapsbriefly, rather than fade into dimness, if the coupling to the pickupcoil is reduced. In the target application this enhances humanattention-gathering by the panel while in other applications this is afail-safe feature for inadvertently reduced power transfer.

[0062] The power section will now be described. Integration of thiscircuit with an inductive pickup secondary circuit (sections (b) and(c)) is preferably as follows: A pickup coil 120, resonant withcapacitor 121, can collect inductive power in the form of a changingmagnetic field and convert it into AC. Typically we generate 10-140 kHzAC in a primary conductor or primary coil 119 to provide a changingmagnetic field. The circulating current in the resonant tank circuit120-121 is rectified in the bridge rectifier 122 and passed through aninductance 123 of typically 560 μH, intended to limit the peak currentpassing through a control switch 124 which is capable of shorting outthe pickup coil (ignoring two diode voltage drops, of course). If theshorting switch is in a high-impedance state, rectified current flowsthrough the steering diode 125 and along the bus 101 to charge up thesmoothing capacitor 126, across which the supply voltage is developed.In this circuit, one comparator determines whether the bus 101 voltageis over 50 volts, in which case its output on line 129 goes high andturns ON the switch 124, or under 49 volts, when its output goes low.This output controls the switch 124. means for enhancing the gap length100 over which inductively coupled inductive power transfer or ICIPT canbe transferred are provided as follows (see both FIG. 1 and FIG. 2). Anat least several times increase of the distance over which effectiveamounts of inductive power can be transferred is attainable. This aspectof the invention comprises placement of a simple intermediate resonantcircuit 117, 118 having (in this example at least) no control means foractive components or the like between a primary source of a changingmagnetic field and the ultimate consumer or consumers. In FIG. 1 thisnovel resonant circuit is represented by the capacitor 118, the lumpedinductance 117, where power is transferred into the circuit from asource of inductive power passing through the inductor 119 which may bea straight conductor or a coil, and the elongated conductor 117A. Inpractice, the elongated conductor might energize a number of pickupssuch as the four tank circuits, one labeled 120 (inductor) and 121(capacitor), each corresponding to a circuit such as that of FIG. 1 andanother labeled 120′ and 121′.

[0063] The physical configuration of the intermediate circuit may becarried according to the application. Electrically it may be representedas inductor 117 and capacitor 118 in FIG. 1. The intermediate pickupcoil can develop higher circulating currents or at least a highermagnetic flux than a primary energized conductor, and so transfer powerover a greater gap length 100. An interesting observation about theintermediate resonant circuit 117/118 is that the overall stability of asystem having extra resonant, tuned circuits of this type is improved.If one circuit (say 120, 121) is detuned away from resonance, its powerfactor changes in use so that for example the current leads the voltage.In a second circuit (say 117 and 118) inductively coupled to the firstthe power factor is reversed so that the voltage now leads the current.In situations where one inductively coupled link exists, the reversalleads to instability, but in situations where a second inductivelycoupled link to a third circuit (119) also exists, the power factorrelationship is again reversed and so the intermediate link leads to animprovement in frequency stability. Of course, this power supply may beused to drive an electroluminescent panel or similar device without theaid of an intermediate loop. FIG. 7 shows such a power supply,practically identical to that of FIG. 1 but with the omission of theloop 117 and 118. Note that in FIGS. 1 and 7 the primary conductivepathway 119 need not have an actual discrete inductance, if sufficientcurrent is flowing in a straight wire to provide an adequate flux.

[0064] Advantages of this power supply include that it renders anelectroluminescent panel a much brighter, and hence more useful device.Furthermore, the increased brightness for a given peak voltage obtainedwith sine-wave driving seems to result in a greatly increased panellife, although the exact improvement of life remains to be defined. Wehad been causing panels and drivers to fail when testing the prior-artsquare-wave driver circuit at higher voltages.

APPLICATION EXAMPLE 2A Road Studs

[0065]FIG. 3 shows a prior-art view of a road stud (which is a deviceincluding a resonant secondary circuit, a control circuit to limitsecondary current, and an array of light-emitting diodes intended as aself-lit “cat's eye” type of lane marker), in which the stud 300 issimply glued onto a road surface 303 above a previously cut slitcontaining primary inductive pathway conductors 304 and 305. Typically,the conductors are spread apart within the space 306 so that themagnetic field surrounding the upper conductor 304 is not partiallycancelled out by the reverse field in 305. Inside the road stud there isa pickup inductor 301 (here shows side-on), resonating circuitry, powercontrol and supply circuitry and a bank of light-emitting diodes 302 toprovide a useful output.

[0066] One problem with this device is that after the road receives eachof an often needed re-sealing the distance over which the inductivefield must travel to reach the stud becomes greater and may exceed thecapability of a given field. (Studs can be hammered free of theiradhesive and perhaps re-used if still functional). The principle of theintermediate loop of the invention is shown in FIG. 4, in which agreater thickness of road surface 303B has been added to the original,302A. One way in which this invention can be used in practice is to cuta further slit in the new seal 303B with a diamond saw or the like, andinsert an intermediate loop 401 made of a good conductor such as a litzwire (which has a high surface area and hence reduced losses at highfrequency) with its resonating capacitor 402. The capacitor may be madas a flat, card-like object rather than the usual cylinder so that itfalls into a narrow slit.

[0067] In laboratory tests, an intermediate resonant circuit comprisinga number of turns of wire wound over two ferrite strips together withappropriate tuning capacitors was made up. This circuit is resonant at asystem frequency and can be placed over a single wire of a conductivepathway. When the circuit is close to the single wire, the road stud canbe excited to a level at which the control circuit becomes operative(perceived as an upper brightness limit) when it is about 10 cm awayfrom the single wire. Without the intermediate resonant circuit, thisdistance is limited to about 3 cm.

[0068] Advantages of this intermediate loop include that the road can bere-sealed more times before the original primary conductive trackwaybecomes useless. The “reach” of the magnetic flux can be extended withthe aid of intermediate loops. We expect that a stack of more than oneintermediate loop will also work although it is possible thatcurrent-limiting means, perhaps a saturable inductor, may be require.

APPLICATION EXAMPLE 2B Two Road Studs

[0069] One road stud close to the primary conductor can act as aintermediate resonant circuit for another road stud; here theintermediate circuit is not a simple passive inductor/capacitor circuitbut is controlled by a shorting switch arrangement for decoupling thecircuit in the event of too high a circulating voltage. This illustratesuse of an intermediate loop incorporating a current limiting feature.Such a feature is useful in permitting primary current to pass anun-loaded intermediate loop and so reach further consuming devices.

APPLICATION EXAMPLE 3A: VEHICLE POWERED THROUGH AN INTERMEDIATE LOOPATTACHED TO THE TRACKWAY

[0070]FIG. 5 shows a vehicle 500, capable for example of running alongan arrangement of rails 501, in which an electric motor 509 drives atlast one wheel; the motor being fed from a set of motor controlcircuitry 508 which accepts AC power from a resonant pickup circuitcomprised of a capacitor 506 and an inductor 507 preferably having aferromagnetic core arranged so as to effectively intercept a flux from asupply. The primary conductor 503 running substantially parallel to thetrack has as an intermediate coupling arrangement a loop of wire(preferably a litz wire, because it may carry a high resonating current)including optionally one or more discrete inductances 504 and a tuningcapacitor 505. The inductances 504 have two functions; they aid incausing the loop to be electrically resonant at a system-wide resonantfrequency, and they act as concentrated sources of inductive fields tobe picked up by the vehicle. In some transport systems there may be anarrangement wherein higher power levels are desirable at certain spots,such as for acceleration. Or, a vehicle may normally be powered byrechargeable batteries 510, to be charged at certain designated spotssuch as “bus stops” along a fixed route. FIG. 5 could in fact beregarded as a sectional view through a bus stop having chargingfacilities. The intermediate loop allows effective charging to occurwith an increased tolerance in the actual rest position.

[0071] Advantages of this invention include that power transfer mayoccur over greater distances. Thus a driver need not be so precise inpositioning the bus over a charging conductor for recharging a busbattery. Reduced vertical positioning constraints allow a vehicle with asofter suspension. A product carrying conveyor device can supplyincreased power where the rails make an ascent. Incidentally, there maybe simple battery charger applications where the increased gap distanceis an advantage and, of course, the constant-current nature of looselycoupled inductive power transfer is an asset in changing batteries. Theintermediate loop may allow increased power to be drawn from a primaryconductor.

[0072] This version of the invention is similar to the arrangement shownin FIG. 6, except that the intermediate loop (including resonatingcapacitor 603, a part of a transformer, and a pickup inductor 507) isnow carried within (or upon) the vehicle and serves a differentfunction. The capacitor 506 and the part-inductor 601 represent theoriginal secondary pickup coil and resonating capacitor. We haveincluded a ferromagnetic core 602 as a convenient way to produce a moreeconomical transformer.

[0073] In this example the advantage of the intermediate loop is that itacts as a system frequency stabilising device. Suppose for example thatthe vehicle resonant pickup comprised of 506 and 601 has shifted awayfrom the system-wide frequency and as a result the phase of the currentwithin the pickup resonant circuit is leading the phase of the voltage.As is well-known, on the other side of a transformer device (includingan inductive pickup device), the phase of the voltage will now tend tolead the phase of the current. This inversion of the order tends tocause system instability. If all intermediate loop is used, then withinthe primary conductor the original leading by the current is restored asresult of passage of the power through a second transformer device—orinductive coupling means in this case. Hence the detuning of the vehicleis less likely to cause system instability; and increased systemstability is resulting advantage. Intermediate loops may be constructedand sold as separate accessories suitable for use with inductive powertransfer systems of various types. One of the variables to be consideredwhen ordering loops is the existing resonant frequency of the systemwith which the intermediate loops arm to be used.

APPLICATION EXAMPLE 4 Beer Tap Handles

[0074] This example integrates all the inventions described in thisspecification. Electrically, the beer tap handle circuit is that shownin FIG. 1, where the luminescent panel 115 is incorporated in the handleof a beer tap to act as an advertising accessory and attract theattention of consumers. In this instance, the reduced coupling thatoccurs when the handle is operated by being pulled away from a restposition has the effect of causing the hitherto steadily lit panel tonot simply go dim, but enter a flashing mode wherein the brightness ofeach flash (which is of a reasonably long duration, depending on thesize of capacitor 126) is comparable to the steady illumination(typically 400 Lux using the circuit of this invention) of the handle inits rest position.

[0075] The components (apart from 119, 117, and 118) of FIG. 1 arepreferably installed in a concealed manner within each handle. Usingsurface-mount electronics size is not a problem. Refer to FIG. 2 for thephysical appearance of an illuminated beer-tap handle for use in a bar.

[0076] The pickup coil (120) comprises perhaps 20 turns on a C-shapedcore which may be cut from a toroidal core. In the example this isoriented vertically (i.e. along the axis of the handle) in order tocollect flux emanating from the intermediate resonant loop 118-117. InFIG. 2, the rectangle 117A may be physically within a panel that passesclose to each beer handle circuit (here suggested by the tank circuits120, 121 etc.). In one corner a concentration of flux pick-up means 117is provided and this is in use oriented close to a source of magneticfield such as a coil 119 driven by a resonant power supply converter(not shown) with a sine wave at typically 40 kHz in this application.The tuning capacitor 118 is generally located close to this coil. Theprimary coil 119 may be incorporated in a clip over the holder for thepanel. In this particular application it is useful to be able to detachthe panel including the coil 117/117A/118 and as there are no directconnections to it, this can be done easily. In fact the componentry canbe totally concealed. The panel can be detachable in this application topermit access to the tap mechanism and for cleaning. The detachablepanel may be cleaned, for it has no active, sensitive parts or exposedelectrical connections.

[0077] When in place, this panel is held so that its particular pickuparea 117 is in proximity to the energizing primary coil, and its borderis near one or more pickup coils, 120, 120′ and so on; one on each beertap.

[0078] Advantages of this system containing an intermediate resonantcircuit for powering electroluminescent panels in a consumer-accessibleand often wet region include (1) that there is no need to placerelatively obtrusive primary coils adjacent to each tap, with wiring,and rigidly held in close relationship to toroids around the beerhandles, so affecting the ability to clean the taps, and (2) that thedistance over which effective coupling occurs is greater, so thatprecise positioning is not required and so that electroluminescentpanels remain lit during movement of the beer tap handles. Several beertaps may be driven from the one intermediate circuit.

VARIATIONS

[0079] For a vehicle, the intermediate loop may be mounted over thefixed primary trackway within the road surface, for gap-wideningpurposes, or it may be mounted within the vehicle in relationship withthe secondary pickup coil or coils, where it serves to increasestability.

[0080] We have not yet explored the operation of a intermediate circuitunder high power operating conditions or where several intermediatecircuits are to be driven simultaneously from a single primaryconductor. Intermediate resonant loops circuits may also need to includecontrol circuits to limit the total circulating power. One possibleexample of a control “circuit” is a saturable ferrite core within thetuning inductance. Another is back-to-back Zener diodes connected acrossthe tuning capacitor, selected so as to break down when the circulatingvoltage exceeds a predetermined limit.

INDUSTRIAL APPLICATION

[0081] (1) Improved drive circuitry for electroluminescent panelsincreases the range of possible applications for such panels. Prior-artdrivers resulted in about 120 lux of emitted light, which is visible butnot dramatic. 400 Lux was available with the circuit of this invention.

[0082] (2) Intermediate resonant circuits, by increasing the gap overwhich a given amount of power can be transferred inductively, canincrease the number of applications for IPT. They can reduce the amountof primary current required, and/or reduce the size of a secondarypickup inductor, or they may reduce the requirements for precisealignment of the pickup coil with the primary conductor. For a roadstud, which is an internally lit “cat's eye” device using light-emittingdiodes, adequate function is obtained when it is about 10 cm away fromthe single wire. Without the intermediate resonant circuit, thisdistance is limited to about 3 cm. The intermediate resonant circuititself is a simple and cheap device.

[0083] Finally, it will be appreciated that various alterations andmodifications may be made to the foregoing without departing from thescope of this invention as set forth in the following claims.

1. An inductive power distribution system including: a primaryconductor, a secondary resonant pick-up circuit for use in conjunctionwith the primary conductor, the pick-up circuit deriving at least somepower from a magnetic field associated with the primary conductor, anintermediate resonant circuit provided between the primary conductor andthe pick-up circuit, the intermediate resonant circuit includinginductance means and capacitance means, the magnitude of the capacitancemeans and the inductance means being chosen so that the intermediateresonant circuit is resonant at a substantially predetermined resonantfrequency, and the presence of the intermediate resonant circuitallowing a given quantity of power to be transferred to the pick-upcircuit across a greater distance between the primary conductor and thesecondary resonant pick-up than distance over which the given quantityof power can be transferred without the presence of the intermediateresonant circuit.
 2. The inductive power distribution system as claimedin claim 1 , wherein the inductance means comprises an inductance whichis provided as a lumped inductance.
 3. The inductive power distributionsystem as claimed in claim 1 , wherein the inductance means comprises aninductance which is a distributed inductance.
 4. The inductive powerdistribution system as claimed in claim 1 , wherein the inductance meanscomprises a lumped inductance and a distributed inductance.
 5. Theinductive power transfer system as claimed in claim 3 , wherein theintermediate resonant circuit comprises a single loop of conductivematerial and the distributed inductance comprises the inductance of theloop.
 6. The inductive power transfer system as claimed in claim 4 ,wherein the intermediate resonant circuit comprises a loop of conductivematerial, and the distributed inductance comprises the inductance of theloop, and whereby the loop includes at least one sub loop having one ormore turns of the conductive material, the sub-loop comprising thelumped inductance.
 7. The inductive power distribution system as claimedin claim 1 , wherein the intermediate resonant circuit includes anelongate loop of conductive material so that a plurality of secondaryresonant pick-up circuits which are spaced apart from each other alongthe length of the elongate loop may draw power from the primaryconductor via the intermediate resonant circuit.
 8. The inductive powertransfer system as claimed in claim 1 , wherein the intermediateresonant circuit includes current limiting means to limit the amount ofresonant current flowing in the intermediate resonant circuit.
 9. Theinductive power distribution system as claimed in claim 8 , wherein thecurrent limiting means includes control means for at least partiallydecoupling the intermediate resonant circuit from the primary conductor.10. The inductive power distribution system as claimed in claim 1 ,wherein the secondary resonant pick-up circuit is provided on anelectrically powered vehicle.
 11. The inductive power distributionsystem as claimed in claim 10 , wherein the intermediate resonantcircuit comprises an elongate loop of conductive material extending overa lateral distance beneath at least a part of a route taken by thevehicle and whereby the intermediate resonant circuit provides power tothe vehicle when the vehicle is located adjacent to the intermediateresonant circuit.
 12. The inductive power distribution system as claimedin claim 2 , wherein more than one lumped inductance is provided withinthe intermediate resonant circuit and the lumped inductance provideregions of increased power transfer to one or more secondary pick-upcircuits.
 13. The inductive power distribution system as claimed inclaim 12 , wherein the secondary pick-up circuits are provided on one ormore vehicles which use the secondary pick-up circuits to charge abattery provided on each vehicle and the lumped inductances in theintermediate resonant circuit provide regions for charging the batteryof each vehicle when the vehicle is located adjacent to the lumpedinductance.
 14. The inductive power distribution system as claimed inclaim 1 , including active frequency adjusting means provided toregulate or adjust the frequency of the resonant intermediate circuit.